Oxidation resistant metallic materials

ABSTRACT

Metallic materials which possess suitable characteristics for serving as an intermediate layer between a thermal barrier coating and a matrix substrate, or as a separator for a solid oxide fuel cell, contain 15 to 40 wt % Cr, 5 to 15 wt % W, 0.01 to 1 wt % M (where M is one or more than two elements chosen from Y, Hf, Ce, La, Nd, and Dy), 0.001 to 0.01 wt % B and a balance of Fe. The average value of the coefficient of thermal expansion of the metallic material is in a range from more than 12.0×10 -6  /K to less than 13.0×10 -6  /K in a temperature range from room temperature to 1,000° C., and the value is close to that of stabilized zirconia. The metallic materials display excellent high temperature oxidation resistance.

FIELD OF THE INVENTION

The present invention relates directly to metallic materials whereof thecoefficient of thermal expansion is approximated to that of stabilizedzirconia. More specifically, the present invention, by forming an alloycomprising either of a Cr-W-M-Fe system or a Cr-W-M-B-Fe system, where Mcan be one or more than two elements of an alloy element groupcomprising of Y, Hf, Ce, La, Nd or Dy, relates to oxidation-resistantmetallic materials exhibiting excellent oxidation resistance at elevatedtemperatures and having an equivalent coefficient of thermal expansionto that of stabilized zirconia. The oxidation-resistant metallicmaterials are appropriate for an intermediate layer between the matrixsubstrate and stabilized zirconia serving as a heat-resistant andcorrosion-resistant coating and solid state electrolyte for the solidoxide fuel cell which has been developed as a third generation fuelcell.

BACKGROUND OF THE INVENTION

Recently, energy conservation and global environmental problems havebeen crucial issues. Accordingly, various high temperature operatingplant equipment including a gas turbine used for power generation have atendency to require higher temperature and higher pressure operation.Because of the current industrial demands as mentioned above, the damageand deterioration of metallic materials being operated under criticaloperational conditions have become to be a serious technical problem.

As a result, in the current type of gas turbines for both aircraft andland power generators, it is a commonly adapted practice to apply acorrosion-resistant coating to the high strength superalloys for rotorblades and stationary blades as well. However, the problems associatedwith damage and/or deterioration problems due to high temperaturecorrosion remain for such coatings.

The principle and basic structure of the coating will be brieflyexplained by taking the thermal barrier coating (TBC) as an example.FIG. 1 is a schematic diagram showing the temperature gradients beingdeveloped entirely through the TBC structure, where A represents aceramic layer, B is an intermediate layer, C is an alloy, and Tgindicates the high temperature combustion gas temperature, while Tarefers to a cooling-air temperature and temperatures T₁, T₂, T₃, T₄, T₅,and T₆ indicate respective temperatures at surface or interface zone.FIG. 2 depicts a cross sectional view of TBC when applied to thecombustion equipment.

The main function of TBC is to prevent a temperature raise in thesurface areas of metallic components by coating a ceramic materialhaving lower thermal conductivity onto metallic components exhibitingthe temperature gradient, as seen in FIG. 1. Application of the TBC tothe gas turbine system, particularly for combustion devices, has beendone for more than 10 years. Recently, application of the TBC to thecooling blades has become a more frequent practice. According to thetests using actual blade, it was recognized that the TBC systemexhibited a thermal barrier effect ranging in temperature from 50° to100° C.

TBC normally comprises a ceramic fusion spray layer and an intermediatefusion spray layer, wherein the former consists mainly of ZrO₂ (a solidsolution with a stabilizing component such as MgO, Y₂ O₃, or CaO) havingmuch lower thermal conductivity of 0.005˜0.006 cal/cm.s.°C. than Al₂ O₃(0.04˜0.08 cal/cm.s.°C.) or TiO₂ (0.01˜0.02 cal/cm.s.°C.), while thelatter intermediate layer comprises of a Ni-Al alloy, Ni-Cr alloy orM-Cr-Al-Y alloy (where M can be Fe, Ni, Co or the like) in order torelieve the thermal expansion difference between the alloy (base)material and thereof, and to enhance the corrosion resistance. Researchhas been conducted to form a multi-layer structure in which theintermediate layer is formed as a mixed layer of metal and ceramicmaterials, or to construct the layer having a completely gradientcomposition.

As a fuel cell is considered as a novel power generating system, thereis a phosphoric acid fuel cell (PAFC) in which the phosphoric acidaqueous solution is used for an electrolyte, a molten carbonate fuelcell (MCFC) using lithium carbonate or kalium carbonate or the like asan electrolyte, and a solid oxide fuel cell (SOFC) using zirconia-systemceramics for an electrolyte. The basic energy conversion of any one ofthe aforementioned fuel cells is based on a direct electric energyconversion from the chemical energy involved in the cells through anelectrochemical reaction. Each of such fuel cells exhibits respectivecharacteristics.

The current issues relating to energy policy and the global environmentsuggest use of fuel cells as soon as possible. Such fuel cells serve asa dispersed power supply which can be installed close to demandlocations or a power supply for the purpose of co-generation. A largelyexpected potential of fuel cells in terms of an amount of needs fordispersed type power supply has been recognized.

Taking the solid oxide fuel cell as an example, its principle and basicstructure will be briefly described by referring to a perspectiveassembly view of the solid oxide fuel cell, as seen in FIG. 3.

As seen in FIG. 3, the solid oxide cell consists of a single cell 4comprising of a fuel electrode (anode) 2 and an air electrode (cathode)3; both of the electrodes are sandwiching both sides of an electrolyteplate 1 of an yttria-stabilized zirconia (YSZ). Moreover, a plurality ofthe single cells 4 is formed in a lamellar structure through a separator5 in order to attain a practically usable power supply. To a passagespace 6 formed between the separator 5 and the fuel electrode (anode) 2,H₂ and CO are supplied as fuel sources. Furthermore, air is supplied toanother passage space 7 being formed between the separator 5 and the airelectrode (cathode) 3.

Referring to FIG. 4 explaining the principle of power generation of thesolid oxide fuel cell, since the main constituent of the town gas as afuel is methane, it needs to be modified to a gas containing mainlyhydrogen at the modifying device 8. Namely, with the modifying device 8,the town gas as a fuel source is modified to hydrogen and carbonmonoxide under a reaction with water vapor and reaction heat which areproduced through a cell reaction. A portion of the reaction product willbe fed to a fuel electrode 2 as a methane.

At the fuel electrode 2, the thus modified hydrogen and carbon monoxidereact with the oxygen ion which is introduced from the air electrode 3through the electrolyte plate 1. At this moment, water and carbondioxide are produced, and the co-generated electrons will be exhaustedto an external circuit 9.

At the air electrode 3, an oxygen ion is generated by the co-generatedelectrons of the external circuit 9 and the oxygen obtained in the air.The oxygen ion is introduced to the fuel electrode 2 through theelectrolyte plate 1.

By progressing reactions taking place at the fuel electrode 2 and theair electrode 3, a direct electric power will be supplied to a load ofthe external circuit 9, for example, an electric bulb.

The aforementioned reaction can be analogous to a application of areverse reaction of the so-called electrolysis reaction of water throughwhich hydrogen is generated on the first electrode surface and oxygen isformed on the second electrode surface if a pair of electrodes areinserted into an aqueous solution containing the electrolyte and acurrent is applied to this system.

A presence of the fusion spray layer made of a stabilized zirconiaappears to be most important and governing the system in said thermalbarrier coating (TBC) structure.

According to the developing plan of the high temperature gas turbine inthe Moonlight Project (which is related to an energy-saving plan, beingpromoted by the Ministry of International Trade and Industry), theachievement has been set in such that (i) the inlet gas temperature is1,773K and (ii) the total power generating efficiency is 55% by aso-called co-generation which can be achieved by a combining with thesteam turbine driven by the heat exhausted from said high temperaturegas turbine.

The current thermal power efficiency is about 40% generated by solelythe steam turbine. If the efficiency can be improved by 10%, it isestimated that, in Japan, the fuel equivalent to approximately 3.1billion dollars can be saved every year.

Although the nickel-based alloy has been utilized in order to achievethe aforementioned specific aim of the higher temperature operation andhigher efficiency, the alloy has only about one year life if it is usedin the gas turbine without any coating thereon. Hence the coatingprotection is definitely indispensable.

However, since there exists a large discrepancy of coefficients ofthermal expansion between the stabilized zirconia (ca. 10˜12˜10⁻⁶ /K)and the Ni-based super alloy (ca. 18˜20×10⁻⁶ /K), the fusion spray layerof the stabilized zirconia is susceptible to cracking. In order to solvethis problem along with the expected better corrosion resistance, anintermediate layer made of Ni-Al alloy, Ni-Cr alloy, or M-Cr-Al-Y alloy(where M can be Fe, Ni, Co, or the like) is sprayed to relieve saiddifferences in coefficients of thermal expansion. However, thecoefficients of thermal expansion of theses alloys is in the order of16˜18×10⁻⁶ /K which is still relatively higher; resulting in aninsufficient result.

The separator is an another important component in the solid oxide fuelcell.

The fuel cell is normally formed as a lamellar structure of plates, asseen in FIG. 3, in order to reduce the internal resistance and toincrease the effective electrode area per unit volume.

Since the coefficient of thermal expansion of the material for theseparator 5 is preferred to be close to those of the air electrode 3,the fuel electrode 2 or said solid state electrolyte 1, better corrosionresistance and high conductivity are also required to materials for theseparator 5, (La, alkaline earth metal)CrO₃ is commonly used for thematerial of the separator 5.

The basic function of said separator 5 is to separate individual singlecells 4 when the single cells are stacked in a lamellar structure, toseal hydrogen gas H₂ and air as fuel sources, and to physically hold theelectrolyte plate 1.

In order to hold the electrolyte plate 1, if the surface area of saidelectrolyte plate 1 is formed with larger than those of the fuelelectrode 2 or the air electrode 3, the lamellar forming can be easilyachieved with the separator 5, resulting in easily holding theelectrolyte plate 1.

However, since the separator 5 is made of brittle ceramic materials asmentioned previously, there are still problems remaining such as a weakstrength and poor formability.

As a material for the separator, several important parameters have to bemet such as strong resistances against both oxidation and reduction andbetter electro-conductivity since the separator connects the airelectrode operated in the high temperature oxidation environment and thefuel electrode operated in the high temperature reduction atmosphere.

Although several materials including LaCr₀.9 Mg₀.1 O₃, CoCr₂ O₄ or Ni-Alalloy have been proposed as a material for the separator, there is atechnical problem such as a poor bonding between these separatormaterials and fuel electrode or the solid state electrolyte.

There is no well-developed technology available to produce and refinethe raw powder with uniform particle distribution for fabricating said(La, alkaline earth metal)CrO₃. Furthermore, although the heat resistingalloys such as stainless steels or Inconel have a superior mechanicalstrength to the aforementioned ceramic materials, the solid stateelectrolyte will be subjected to a tensile stress at a cell operatingtemperature (which is about 1,000° C.) due to its relative largecoefficient of thermal expansion. Moreover, the above-mentioned alloyspossess another problem associated with a large electric resistancecaused by the oxide film formed on its surface.

As to the metallic separator, there are two major problems; one is athermal mismatch with regard to the differences in coefficients ofthermal expansion and the other is a thickening of oxide film formed onthese heat resistant steels. Regarding the problem associated with thecoefficient of thermal expansion, there are several trials done; forexample, (i) using a foam foam structure of the LaMnOx as a connector,(ii) approximating the coefficient of thermal expansion by controllingthe alloy compositions, or (iii) spraying LaCrO₃ to prevent furthergrowing of the surface oxide film. Unfortunately, these trails are notsatisfactory.

Presently, because of excellent characteristics including the highstrength and high toughness, high melting point and heat insulation, aswell as some other electrical properties; the zirconia stabilized withvarious stabilizing components such as MgO, Y₂ O₃, CaO or the like asits solid solution has been a main research/development objective tofind better applications and the establishment of a production processwith the appropriate selection of stabilizing components. Hence thestabilized zirconia has been utilized in versatile fields in industriesincluding a steel industry, chemical industry, cells, fusion materials,turbines, internal combustion engine, sensors and others. In most of theapplications, the stabilized zirconia is employed in a fashion that itis close to or adhered to metallic materials. However, a metallicmaterial, which has an equivalent coefficient of thermal expansion tothat of ceramic and hence can be used for different applications, hasnot been proposed yet.

OBJECTIVE OF THE INVENTION

After observing that, in the prior art, there is no metallic materialproposed having an equivalent coefficient of thermal expansion to thatof the stabilized zirconia and an excellent oxidation resistance; it istherefore an objective of the present invention to produce a metallicmaterial having an excellent oxidation resistance, which has a suitableproperty for serving as an intermediate layer between said thermalbarrier coating and the matrix substrate or as a separator material usedin the solid oxide fuel cell. Moreover, said metallic material has aclose value of the coefficient of thermal expansion to that of thestabilized zirconia.

SUMMARY OF THE INVENTION

After a continuous and diligent effort in developing metallic materialhaving an excellent oxidation resistance and an equivalent coefficientof thermal expansion to that of the stabilized zirconia, it was foundthat an Fe-based alloy containing a certain amount of Cr, W and M (whereM can be one or more than two elements chosen from a group comprising Y,Hf, Ce, La, Nd, or Dy) has a close value of coefficient of thermalexpansion to that of the stabilized zirconia and exhibits an excellenthigh temperature oxidation resistance.

Moreover, it was also found that a grain boundary segregation of Welement can be avoided by adding a small amount of B element to theCr-W-M-Fe alloy.

As an alloy containing Cr, W and Fe, a ferritic steel or the like hasbeen known as a stainless steel used as the gas turbine or boiler tubes(Japan Patent Publication No. Sho 57-45822, Japan Patent Publication No.Hei 3-59135, Japan Patent Publication No. Hei 3-65428, Japan PatentPublication No. Hei 4-54737, Japan Patent Publication No. Hei 5-5891,and Japan Patent Application Laid-Open No. Hei 2-290950).

However, all of these alloys disclosed in the aforementioned patentapplications were developed only to enhance the high temperaturestrength. There was no consideration made in these applications on thecoefficient of thermal expansion.

Furthermore, the alloy compositions of the aforementioned alloys(namely, Cr:7.0˜15.0 wt %, W:0.05˜3.5 wt %) are quite different fromthose proposed in the present invention.

Namely, according to the present invention, the oxidation resistingmetallic material has the following composition; that is, Cr:15˜40 wt %,W:5˜15 wt %, one or more than two elements of Y, Hf, Ce, La, Nd orDy:0.01˜1 wt % or B:0.001˜0.01 wt %, Fe: balance, along with unavoidableimpurities. Moreover, the metallic material of the present inventionexhibits an average coefficient of thermal expansion of more than12×10⁻⁶ /K and less than 13×10⁻⁶ /K in a temperature range from the roomtemperature to 1,000° C.

The above and many other objectives, features and advantages of thepresent invention will be more fully understood from the ensuingdetailed description of the preferred embodiment of the invention, whichdescription should be read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a temperature gradient entirelycross over the thermal barrier coating (TBC) system.

FIG. 2 is a microstructure depicting a cross sectional structure of thethermal barrier coating (TBC).

FIG. 3 is a perspective assembly view showing a structure of a solidoxide fuel cell.

FIG. 4 shows an operational principle of a solid oxide fuel cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, Cr is a basic alloying element toexhibit a heat resistance and is needed to contain at least 15 wt %.However, if it contains more than 40 wt %, its beneficial effect will besaturated, rather it will cause the coefficient of thermal expansionincrease and reduce the formability. Hence it should be in a range of 15to 40 wt %, preferably it should be in a range of 15 to 25 wt %.

W is another basic alloying element to obtain a certain value ofcoefficient of thermal expansion; hence it should be included at least 5wt %. However, if it exceeds more than 15 wt %, the coefficient ofthermal expansion will increase. As a result, it should be in a range of5 to 15 wt %. Preferably, it should be included in a range of 5 to 10 wt%.

Element group consisting of Y, Hf, Ce, La, Nd and Dy can enhance theoxidation resistance by either single element alloying or mixed elementalloying. It is preferred to contain it at least 0.01 wt %. However, ifit exceeds 1 wt %, the hot-formability will be rapidly deteriorated.Hence, it is preferable to contain it at a range of 0.01 to 1 wt %.

It is known that B element is an effective alloying element to prevent Welement from the grain boundary segregation. Hence it is required tocontain at least 0.001 wt %. However, if it contains more than 0.01 wt%, its beneficial effect will be saturated; therefore it is preferablyto contain in a range of 0.001 to 0.01 wt %.

Fe serves a matrix element of the metallic material of the presentinvention and contains as a balance for this alloying system.

The metallic material of the present invention can be fabricated througha prior art casting technique. The thus-obtained ingot can be furthersubjected to hot- or cold-working or using the powder thereof tomanufacture the final products which should be suitable for variouspurposes.

The mechanical properties and the heat-resistance of the metallicmaterial of the present invention possess equivalent characteristics tothose found in conventional types of stainless steels.

The value of the coefficient of thermal expansion of the metallicmaterial of the present invention, because of an equivalent value to thecoefficient of thermal expansion of the stabilized zirconia (i.e.,10˜12×10⁻⁶ /K), is defined within a range from more than 12.0×10⁻⁶ /K toless than 13.0×10⁻⁶ /K in a temperature range from the room temperatureto 1,000° C.

Since the presently invented metallic material has a close value ofcoefficient of thermal expansion to that of stabilized zirconia (i.e.,10˜12×10⁻⁶ /K) and shows an excellent oxidation resistance, it can serveas an intermediate layer between the heat-resistant and corrosionresistant coating layer, or as a material for separator in a solid oxidefuel cell. As a result, the metallic material according to the presentinvention exhibits a suitable characteristic in such a way when utilizedalong with the stabilized zirconia or with the material having a closevalue of coefficient of thermal expansion of that of said stabilizedzirconia.

EMBODIMENT 1

The alloy having chemical compositions as listed in Table 1 wasfabricated and the coefficient of thermal expansion at a temperaturerange from the room temperature to 1,000° C. and the weight gain due tothe high temperature oxidation were measured. The results obtainedthrough these measurements are listed in Table 2 along with the resultsobtained from the comparison materials.

From Table 2, it is observed that the value of coefficient of thermalexpansion of the present metallic material showed a close value to thatof the stabilized zirconia (that is, 10˜12×10⁻⁶ /K). Moreover, theexcellent oxidation resistance of the presently invented metallicmaterial is also recognized.

For evaluation of the oxidation resistance, the weight gain due to thehigh temperature oxidation by 1,000° C.×1,000 hours in air was obtainedby subtracting the weight prior to oxidation tests, and the obtainedweight difference (eventually weight gain) was divided by the totalexposed surface areas.

Moreover, although samples No. 2, 4, 5, 6, 8 and 9 which do not containB alloying element showed a slight evidence of grain boundarysegregation of W element, no evidence of the grain boundary segregationof W was found with B-containing alloys No. 1, 3 and 7.

INDUSTRIAL APPLICABILITY

The metallic material comprising Cr-W-M-Fe or Cr-W-B-M-Fe alloy (where Mcan be one or more than two elements from a group consisting of Y, Hf,Ce, La, Nd and Dy) with a certain chemical compositions, according tothe present invention, has a closer value of the coefficient of thermalexpansion to that of the stabilized zirconia than that of theconventional stainless steel, and shows excellent high temperatureoxidation resistance; so that it is most suitable to be used as anintermediate layer between the heat resistant coating layer made of thestabilized zirconia and corrosion resistant coating layer made ofstabilized zirconia or as a separator for a solid oxide fuel cell inwhich the stabilized zirconia is employed as a solid state electrolyte.

While this invention has been described in detail with respect topreferred embodiment and examples, it should be understood that theinvention is not limited to that precise embodiments; rather manymodifications, and variations would present themselves to those of skillin the art without departing from the scope and spirit of thisinvention, as defined in the appended claims.

                                      TABLE 1                                     __________________________________________________________________________           Sample                                                                            Composition(Weight %, Balance Fe)                                         No. Cr W  Y  Hf Ce La Nd Dy B                                          __________________________________________________________________________    This Invention                                                                       1   17.6                                                                             7.1                                                                              -- 0.08                                                                             -- -- -- -- 0.003                                             2   17.8                                                                             9.8                                                                              0.95                                                                             -- -- -- -- -- --                                                3   17.5                                                                             14.9                                                                             -- -- -- 0.32                                                                             -- -- 0.009                                             4   18.0                                                                             5.2                                                                              -- -- 0.155                                                                            0.065                                                                            -- -- --                                                5   15.1                                                                             8.0                                                                              -- -- -- -- 0.16                                                                             -- --                                                6   20.0                                                                             6.8                                                                              -- -- 0.008                                                                            0.002                                                                            -- -- --                                                7   24.2                                                                             5.1                                                                              -- -- -- -- 0.30                                                                             0.05                                                                             0.005                                             8   34.5                                                                             7.4                                                                              -- -- 0.018                                                                            0.005                                                                            -- -- --                                                9   39.7                                                                             6.5                                                                              -- -- -- -- -- 0.12                                                                             --                                         Comparison                                                                           10  17.3                                                                             -- -- -- -- -- -- -- --                                                11  17.8                                                                             7.0                                                                              -- -- -- -- -- -- 0.001                                             12   5.1                                                                             6.8                                                                              -- -- -- -- -- -- --                                                13  10.2                                                                             7.1                                                                              -- -- -- -- -- -- --                                                14  44.8                                                                             7.2                                                                              -- -- -- -- -- -- --                                                15  13.5                                                                             16.1                                                                             -- -- -- 0.17                                                                             -- -- --                                         __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                                                Weight                                                             Coefficient of                                                                           gain                                                               Thermal    due to                                                             Expansion  oxidation                                                    Sample                                                                              (R.T.      (mg/                                                         No.   ˜ 1000° C.)                                                                 cm.sup.2)                                                                              Notes                                        ______________________________________                                        This Invention                                                                         1       12.1 × 10.sup.-6 /K                                                                1.3    no grain boundary                                                             segregation of W                                    2       12.8 × 10.sup.-6 /K                                                                3.0                                                        3       12.9 × 10.sup.-6 /K                                                                1.9    no grain boundary                                                             segregation of W                                    4       12.4 × 10.sup.-6 /K                                                                0.8                                                        5       12.2 × 10.sup.-6 /K                                                                2.5                                                        6       12.4 × 10.sup.-6 /K                                                                1.1                                                        7       12.7 × 10.sup.-6 /K                                                                2.3    no grain boundary                                                             segregation of W                                    8       12.5 × 10.sup.-6 /K                                                                1.6                                                        9       12.9 × 10.sup.-6 /K                                                                2.8                                               Comparison                                                                             10      13.8 × 10.sup.-6 /K                                                                22.6   AlSI Type 430                                       11      12.0 × 10.sup.-6 /K                                                                7.3                                                        12      13.6 × 10.sup.-6 /K                                                                30.3                                                       13      13.3 × 10.sup.-6 /K                                                                26.7                                                       14      13.2 × 10.sup.-6 /K                                                                5.2                                                        15      13.5 × 10.sup.-6 /K                                                                6.4                                               ______________________________________                                    

We claim:
 1. An oxidation resistant metallic material exhibiting anaverage coefficient of thermal expansion ranging from more than 12×10⁻⁶/K to less than 13×10⁻⁶ /K in a temperature range from room temperatureto 1,000° C., consisting essentially of 15 to 40 wt % Cr, 5 to 15 wt %W, 0.01 to 1 wt % of at least one element chosen from the groupconsisting of Y, Hf, Ce, La, Nd and Dy, balance Fe and unavoidableimpurities.
 2. An oxidation resistant metallic material exhibiting anaverage coefficient of thermal expansion ranging from more than 12×10⁻⁶/K to less than 13×10⁻⁶ /K in a temperature range from room temperatureto 1,000° C., consisting essentially of 15 to 40 wt % Cr, 5 to 15 wt %W, 0.001 to 0.01 wt % B, 0.1 to 1 wt % of at least one element chosenfrom the group consisting of Y, Hf, Ce, La, Nd and Dy, balance Fe andunavoidable impurities.
 3. The metallic material according to claim 1,consisting of 15 to 40 wt % Cr, 5 to 15 wt % W, 0.01 to 1 wt % of atleast one element chosen from the group consisting of Y, Hf, Ce, La, Ndand Dy, balance Fe and unavoidable impurities.
 4. The metallic materialaccording to claim 2, consisting of 15 to 40 wt % Cr. 5 to 15 wt % W,0.1 to 1 wt % of at least one element chosen from the group consistingof Y, Hf, Ce, La, Nd and Dy, balance Fe and unavoidable impurities.